WASHINGTON — If President Obama were to decide to leave no military advisory force in Afghanistan next year, would Afghan security unravel to the point of enabling a civil war, a Taliban takeover and a return of al-Qaida in such numbers as to pose a 9/11-type threat?

That is the question at the core of the debate over Obama’s next move in a long-running standoff with the Afghans over a postwar presence.

A look at the debate and the arguments behind it.

Q: Hasn’t Obama said the war will end this year?

A: Yes, but that tells only part of the story of the endgame for America’s longest war. Obama refers to ending the U.S. and coalition combat role in Afghanistan on Dec. 31, as was agreed four years ago. Aside from the fact that for Afghans, the war and its misery almost certainly will not end this year, the remaining issue for Obama is whether to begin a follow-on military advisory mission in January that also would allow U.S. special operations forces to continue hunting the small remaining numbers of al-Qaida in eastern Afghanistan.

Q: What good would advisers do, if 13 years of combat couldn’t win the war?

A: The thinking is that with a little more help, U.S.-trained Afghan government troops could at least hold their own against the Taliban over the next few years, creating a better chance for a long-term political settlement. It has been clear for some time that U.S. and coalition forces can damage but not decisively defeat the Taliban, and that the best hope for peace and stability is to enable an Afghan solution.

A study by CNA Strategic Studies, a federally funded think tank, published earlier this month concluded that the Taliban is likely to regroup and gain strength after 2014, posing a threat to the government in the 2015-18 period, even if U.S. and other international advisers remain. But the picture could grow much worse, the study said, if foreign forces abandon the country entirely. It predicted a “downward spiral” of Afghan security force capabilities, and of Afghan security, unless U.S. and other advisers remain and the international community continues to underwrite the cost of Afghan’s army.

Q: So what’s the argument against keeping advisers there?

A: The practical problem is that Afghan President Hamid Karzai has thus far refused to sign a deal he negotiated with Washington last year that provides the legal basis for U.S. forces to remain there after 2014. No deal, no troops, the White House says. There also are political factors. A war-weary American public appears to have little appetite for even a slimmed-down commitment to remaining involved in the inconclusive war, which began in 2001 and is the longest in American history.

Q: Is Obama likely to abandon the idea of providing a follow-on advisory force?

A: He is keeping his options open, but many who have closely followed the war believe Obama will keep pressing the Afghans to approve a U.S.-NATO advisory mission. Less than a year after he took office, Obama took ownership of the war by ordering an extra 30,000 troops into battle, saying America’s security depends on a stable Afghanistan.

Michael O’Hanlon of the Brookings Institution think tank said in an email interview Thursday that by choosing the “zero option,” as it has come to be called, Obama would be “perilously close to conceding — or at least risking — defeat” in Afghanistan. He does not believe Obama is leaning toward that option.

Q: How is it possible that, after 13 years of war, the Afghan forces are not ready to stand on their own?

A: It depends on how you look at it. U.S. officials say they believe the Afghan army has shown it can stand up to the Taliban. Afghan forces have taken the lead combat role for many months now. But they have enjoyed the luxury of knowing that American troops have their backs. The problem is not the Afghans’ will or ability to fight. The problem, from a purely military standpoint, is the government’s limited ability to sustain them in that fight. In other words, the Afghans need help running their own defense establishment to keep weapons and supplies — and paychecks — flowing to the troops, and to develop effective air forces.

Defense Secretary Chuck Hagel said Thursday in Brussels that without U.S. and coalition backup, there is a risk of an “erosion of confidence” among Afghan troops. Some say that could lead to their collapse. The U.S.’s top military officer, Joint Chiefs of Staff Chairman Gen. Martin Dempsey, said after meeting with ground commanders in Afghanistan on Wednesday that he feared a Western departure would embolden the Taliban and even cause some Afghan troops to cooperate with the Taliban to “hedge their bets.”

Q: When will Obama have to decide whether to pull all U.S. troops out?

A: There is no hard deadline. Administration officials had said late last year that they needed a decision by Karzai in a matter of weeks. By it now appears that Obama could afford to wait even beyond this summer. By July, the top U.S. commander in Kabul, Gen. Joseph Dunford, expects to have established a force tailored for a post-2014 advising and counterterrorism mission, although the military has said it would like to know by early summer, in part because some NATO allies who want to be part of that post-2014 force need time to make their own arrangements. It appears Dunford might be able to wait as late as October for a decision on whether to fully withdraw.